Christian Peeters and Bert Hölldobler's Experiments on Reproduction in Indian Jumping Ants (1991–1994)

Christian Peeters and Bert Hölldobler's Experiments on Reproduction in Indian Jumping Ants (1991–1994)

Between 1991 and 1994, Christian Peeters and Bert Hölldobler
studied the reproductive behaviors of the Indian jumping ant
(Harpegnathos saltator), a species native to southern India. They
conducted experiments as part of a larger investigation into conflict
and reproductive behavior among ants. Peeters and Hölldobler
discovered that Indian jumping ant colonies contained both sexually
reproductive workers and egg-laying queens. In most other species of
ant, the queens are the only sexually reproductive individuals. After
conducting their experiments, Peeters and Hölldobler argued that
queens and sexually reproductive workers cooperated in the Indian
jumping ant species to establish and preserve new colonies.

Peeters
and Hölldobler began working together at the University of Würzburg in
Würzburg, Germany. Peeters studied zoology at the University of
Witwatersrand in Johannesburg, South Africa, and joined Hölldobler's
ant research team at the University of Würzburg as a post-doctoral
fellow in 1991. Hölldobler won the Gottfried-Wilhelm-Leibniz Prize in
1990, a large research grant from the German Science Foundation, and
he used the money to hire early career researchers like Peeters.
Peeters and Hölldobler began collaborating on a series of academic
articles about conflict within ant colonies.

Peeters and Hölldobler
investigated the behaviors of the two types of ant that make up an ant
colony, worker ants and queen ants. Queen ants are physically
specialized to reproduce, and though multiple egg-laying queens
sometimes coexist in a single colony, researchers at the time stated
that most ant species lived in single queen colonies. Queen ants lay
eggs that can develop into worker ants, male ants, or queen ants.
Worker ants almost never reproduce, and are always female. Male ants
and queen ants, the two types of ant physically specialized for
reproduction, only develop once a year just before mating season. Male
ants inseminate virgin queens during mating season and die shortly
after. The newly inseminated queens then go on to found new ant
colonies by laying eggs that develop into workers.

Peeters and
Hölldobler studied how queen ants inhibited workers from laying eggs.
As Hölldobler explained in his 1990 book The Ants, queens prevent
workers from laying eggs in multiple ways. The queens of many ant
species exude a chemical that inhibits workers from producing eggs,
and the queens of some ant species physically attack fertile workers.

As part of their investigation into the reproductive behaviors of
workers and queens, Peeters and Hölldobler began studying ants in the
Ponerinae subfamily. Ants in the Ponerinae subfamily are different
from other subfamilies of ant because Ponerine ants are
phylogenetically primitive, meaning they have more characteristics in
common with their hypothesized wasp ancestors than other living
subfamilies of ant. Though the workers of most ant species lack the
sexual organs needed to mate with males and produce eggs, Peeters
noted that the workers of some Ponerine species retained those sexual
organs. Peeters also noted that queen ants in the Ponerinae subfamily
tended to produce fewer eggs than the queens of other subfamilies. As
a result, Ponerine colonies tend to be small relative to the colonies
of other subfamilies. One of the Ponerine species studied by Peeters
and Hölldobler had an average colony size of fifty-four ants, for
instance. The relatively small size of Ponerine ant colonies paired
with the fertility of Ponerine enabled Peeters and Hölldobler to more
easily study ant reproductive behaviors.

During the early 1990s,
Peeters and Hölldobler performed experimental work on a Ponerine ant
species often called the Indian jumping ant. Peeters and Hölldobler
excavated forty-four Indian jumping ant colonies in Karnataka, India,
between 1991 and 1994. Peeters and Hölldobler counted the number of
workers and queens in each colony. To determine the number of
inseminated, egg-laying workers, Peeters and Hölldobler dissected the
sexual organs of 865 workers sampled from fifteen of the forty-four
excavated colonies. Peeters and Hölldobler determined whether or not a
worker had been inseminated by testing the worker's sexual organs for
sperm. They determined whether or not a worker had been laying eggs by
checking the worker's sexual organs for yellow bodies. Yellow bodies
are accumulations of a specific kind of cell that eggs deposit as they
leave the body of an egg-laying ant. The more yellow bodies Peeters
and Hölldobler found inside a worker, the more eggs that worker had
been laying.

Having compiled their data, Peeters and Hölldobler found
that inseminated workers were common among Indian jumping ants. Of the
fifteen colonies Peeters and Hölldobler sampled, fourteen of those
colonies contained at least one inseminated worker. Seven of those
fifteen colonies had no queens, making workers the only egg-layers in
those colonies. Six of the fifteen colonies contained a queen who was
the sole egg-layer of the colony. Of those six colonies in which
queens were the only egg-layers, five contained inseminated workers
that had not laid any eggs. Finally, in two of the fifteen colonies
sampled, Peeters and Hölldobler found egg-laying workers living
alongside an egg-laying queen.

From that data, Peeters and Hölldobler
inferred some of the reproductive practices of queens and workers.
Because Peeters and Hölldobler found inseminated workers living
alongside queens, they concluded that workers were mating with males
while the queens were alive. The researchers argued that inseminated
workers laid no eggs in most of the colonies that still contained a
queen, because the queens inhibited the workers' ovaries from
producing eggs. In those two colonies for which Peeters and Hölldobler
found workers laying eggs despite the presence of a queen, Peeters and
Hölldobler discovered that the egg-laying workers had a small amount
of light colored yellow bodies in their sexual organs, meaning the
workers had only recently begun laying eggs. Peeters and Hölldobler
interpreted that result as evidence for the claim that Indian jumping
ant workers slowly begin laying eggs as the queen nears the end of her
life and becomes a less productive egg-layer.

To explain the
reproductive practices of Indian jumping ants, Peeters and Hölldobler
articulated a new hypothesis for the lifecycle of Indian jumping ant
colonies. An Indian jumping ant colony, Peeters and Hölldobler argued,
begins with a single inseminated queen founding a colony. Only queen
Indian jumping ants can start new colonies. The queen then produces
workers to populate the new colony while producing males and virgin
queens once a year in preparation for mating season. During mating
season, the males and virgin queens leave the nest to mate with the
males and virgin queens of other colonies. However, instead of mating
exclusively with virgin queens, some males mate with workers from
other colonies. Those inseminated workers do not lay eggs until their
queen dies or nears the end of her life. Once the queen dies, the
inseminated workers begin laying enough eggs to sustain the colony and
to produce males and virgin queens during mating season. According to
Peeters and Hölldobler, when an Indian jumping ant colony contains
only workers, it is in the final stage of its lifecycle.

Finally,
Peeters and Hölldobler provided a hypothesis for why the reproductive
practices of Indian jumping ants differ from the reproductive
practices of other ant species in which workers never reproduce.
According to Peeters and Hölldobler, Indian jumping ant queens do not
fully inhibit workers from reproducing because Indian jumping ants
have evolved a set of behaviors that enables a colony to transition
from one egg-laying queen to multiple egg-laying workers. Peeters and
Hölldobler argued that Indian jumping ants have evolved that set of
behaviors because the nests Indian jumping ants build require too much
work from the ants to build them for those ants to abandon them every
time a queen dies. Unlike other Ponerine ant species, Indian jumping
ants construct elaborate underground nests to survive in the flood
prone plains of southern India. Peeters and Hölldobler concluded that
Indian jumping ants evolved an unusual set of reproductive behaviors
to get more use out of the significant labor investment that their
nests represent.

Peeters and Hölldobler published the results of their
research in a 1995 article titled "Reproductive Cooperation between
Queens and Their Mated Workers: The Complex Life History of an Ant
with a Valuable Nest." Researchers generally accepted Peeters and
Hölldoblers' explanation of the Indian jumping ant's reproductive
cycle. Academic articles attempting to synthesize data about the
reproductive practices of ants have cited Peeters and Hölldoblers'
article as documenting a rare reproductive scheme in ants. Other ant
researchers, and Peeters and Hölldobler, continued to study the
reproductive practices of Indian jumping ants by designing experiments
to uncover the role of chemical communication in Indian jumping ant
reproductive practices.